岩石铷-锶和钐-钕同位素标准物质的研制

通常样品的⁸⁷Sr/⁸⁶Sr和¹⁴³Nd/¹⁴⁴Nd同位素比值分析采用SRM9⁸⁷、JNdi-1作为标准物质,它们分别是纯的碳酸盐和氧化物,适用于监控质谱测试过程。中国现有的钐-钕地质和铷-锶年龄标准物质,分别为玄武岩和钾长石,它们与很多地质样品的基质存在差别。仅有这两种基质的标准物质不能有效地监控不同地质样品Rb-Sr、Sm-Nd同位素分析过程,因此研制不同岩性的Rb-Sr、Sm-Nd同位素标准物质具有重要现实意义。本文采集中国典型地区的橄榄岩、榴辉岩和花岗岩作为候选物,严格按照《一级标准物质技术规范》(JJF 1006-1994)和《标准物质定值的通用原则及统计学原理》(JJF 1343-2012)等相关标准物质国家计量技术规范和国家标准,研制了橄榄岩、榴辉岩和花岗岩铷-锶、钐-钕同位素标准物质(编号为GBW04139、GBW04140、GBW04141),其中橄榄岩标准物质适用于高Mg、Fe,低Rb、Nd含量样品的分析,榴辉岩和花岗岩标准物质适用于含有难溶副矿物的岩石样品的分析。每个标准物质具有6个特性量值,Rb、Sr、Sm和Nd含量分布分别为0.16~64μg/g、12~560μg/g、0.1~3.2μg/g和0.3~15.3μg/g,⁸⁷Sr/⁸⁶Sr比值分布为0.70446~0.71309,¹⁴³Nd/¹⁴⁴Nd比值分布为0.51115~0.51267,同位素比值精度达到或优于同类标准物质。这些特性量值更接近实际样品,使用时将更加有效和方便。该系列标准物质可用于校准仪器和评价方法,并能有效监控实验室此类样品的铷-锶、钐-钕同位素分析过程。

Preparation of the Reference Materials for Rb-Sr and Sm-Nd Isotope Analysis

BACKGROUND:One Rb-Sr certified reference materials(GBW04411)and one Sm-Nd certified reference materials(GBW04419)for geological age,which are potash feldspar and basalt respectively,were produced and certified about 30 years ago.They have only one certified value and their matrices differ from many other geological samples.To better assess the quality of chromatographic separation,measurement procedures and mass spectrometry performance when analyzing Rb-Sr and Sm-Nd,three CRMs which are peridotite,eclogite and granite for Rb-Sr and Sm-Nd analysis were produced,and labelled GBW04139,GBW04140 and GBW04141 respectively.OBJECTIVES:To prepare certified reference materials for analysis of Rb-Sr and Sm-Nd isotopes of different types of rocks.METHODS:Peridotite,eclogite and granite from typical areas in China were used as candidates.Referring to‘Determinations for isotopes of lead,strontium and neodymium in rock samples(GB/T 17672—1999)’,‘Determination of Rb-Sr isotopic geological age and Sr isotope ratio in rocks and minerals(DZ/T 0184.4—1997)’,‘Determination of Sm-Nd isotopic geological age and Nd isotope ratio in rocks and minerals(DZ/T 0184.6—1997)’,Rb,Sr,Sm and Nd were purified by chromatographic separation,and their concentrations were analyzed by ID-TIMS,⁸⁷Sr/⁸⁶Sr and¹⁴³Nd/¹⁴⁴Nd ratios were determined by TIMS and MC-ICP-MS,respectively.RESULTS:For homogeneity testing of the three CRMs,fifteen bottles of each were randomly selected(from the 200 bottles prepared)for analysis.F-testing was used to study homogeneity.The result was insignificant(187Sr/⁸⁶Sr,¹⁴³Nd/¹⁴⁴Nd.Given that|b₁|0.95,3×s(b₁),the slope was insignificant and no instability was observed.The certified value was calculated from the unweighted means of the results submitted by the participating laboratories,including Institute of Geology,Chinese Academy of Geological Sciences;Institute of Geology and Geophysics,Chinese Academy of Sciences;Tianjin Center,China Geological Survey;School of Earth and Space Sciences,University of Science and Technology of China;Wuhan Center,China Geological Survey;Beijing Research Institute of Uranium of Geology;School of Earth Sciences and Engineering,Nanjing University;First Institute of Oceanography,State Oceanic Administration of China;and National Research Center for Geoanalysis.Uncertainties associated with batch characterisation(uchar),possible between-bottle variations(ubb or ubb’)and those derived from effects related to long-term storage(us)can be expressed as standard uncertainties and combined as follows:U_CRM=√u_char²+u_bb²+u_s²A coverage factor of k=2 was used.Certified values and uncertainties of GBW04139,GBW04140 and GBW04141 were shown in the following table.The accuracy of isotope ratio reached or was better than that of similar standard materials.CONCLUSIONS:All the rocks used for CRMs were collected from typical regions of Peridotite,Eclogite and Granite,the matrix were consistent with the geological samples.The three CRMs can meet the requirements of Rb-Sr and Sm-Nd analysis for rock samples.